It is well-known that metal castings having complex or intricate shapes may be obtained using plastic foam moldings, embedded in a support material such as sand, which vaporize upon contact with the molten metal. The molten metal replaces the vaporized plastic foam, thus creating a metal replica of the molding in the support medium.
However, the plastic foam moldings are typically prepared using thermoplastic polymers having relatively low melting or softening points. For example, polystyrene has an ignition point of 380.degree. C. but a softening point of only about 100.degree. C. At a temperature of about 100.degree. C., a polystyrene foam molding begins to shrink and crumble. This lack of heat resistance can adversely affect the quality of the metal casting obtained therefrom. The problem is aggravated by the relatively high ambient temperatures present in an industrial foundry setting.
Additionally, the rate of pyrolysis of a plastic foam molding is generally not readily controllable. Due to this lack of control, defects in the metal castings resulting from the escaping gas generated by the vaporized foam cannot be easily eliminated. It is thus apparent there is a great need for a method whereby the heat resistance and pyrolysis rate of a plastic foam molding can be adjusted as desired in response to the particular requirements of an evaporative pattern casting process.